Orthogonal Frequency Division Multiplexing (OFDM) has been selected for the air-interface in many communication systems, e.g. in 3rd Generation Partnership Project Evolved UMTS Terrestrial Radio Access (3GPP E-UTRA). These communication systems can facilitate high spectral efficiency on each radio link, since OFDM is suitable for combination with Multiple Input Multiple Output processing (MIMO-processing) and opportunistic transmission schemes.
However, the spectral efficiency of the system also depends on the power level of the out-of-band emission, i.e. the power level of the OFDM signal being emitted outside a designated transmission bandwidth. The out-of-band emissions must be kept below certain levels in order not to cause significant interference in adjacent frequency bands. If the out-of-band power is efficiently suppressed, adjacent frequency channels can be spaced densely and thereby the system's spectral efficiency is improved.
For these reasons, in many system standards, the out-of-band power emissions are regulated and restricted. Several types of requirements exist to regulate the out-of-band power emissions of a signal. In E-UTRA for example, spectral masks, adjacent-channel-leakage-ratios and occupied bandwidth requirements have been defined.
An OFDM signal (being a multiplex of a large number of finite-length complex exponentials) has a power spectrum determined by a number of squared-sinc-shaped functions. Typically, due to the finite length of the exponentials, OFDM signals will not meet a standard's requirements on out-of-band emission, since the spectrum sidelobes decay slowly. This slow decay causes the OFDM power spectrum to become relatively broad, resulting in problematic out-of-band emissions, which have to be reduced in some way.
The power spectrum of an OFDM signal is determined by two quantities; a pulse shape and a correlation between the transmitted symbols. In the prior art, two categories of methods for reducing out-of-band emission have been developed, each of these two categories dealing with one of these quantities of the OFDM signal.
When all data symbols in the OFDM signal are uncorrelated, the slow decay of the OFDM spectrum is caused by the finite-duration property of the pulse shape for the individual OFDM symbols constituting the OFDM signal. The finite-duration causes here instantaneous changes in the phase and amplitude of the signal at the edges of the OFDM symbols. In other words, out-of-band power emanates from the signal transitions between different OFDM symbols.
In prior art, time-windowing of the OFDM signal has been proposed in order to tie consecutive OFDM symbols together. This method belongs to the first category mentioned above, i.e. it changes the pulse shape, and uses a prolonged cyclic prefix and an additional postfix. By overlapping a time-windowed postfix of a previous symbol with a time-windowed cyclic prefix of a current symbol, a continuous transition among the two symbols is here achieved. However, due to the use of a longer cyclic prefix used by the method, the symbol rate and/or spectral efficiency of the system decreases when the method is implemented.
Also, time-windowing could be performed without overlapping the two consecutive OFDM symbols. This variant can be regarded as a ramping in the front and end of the OFDM symbol, forcing its beginning and end to the same point, namely zero. However, the ramping method results in a shorter effective cyclic prefix, and thus also in a higher sensitivity to channel dispersion.
Further, in some prior art solutions belonging to the second category mentioned above, i.e. introducing correlation between transmitted data symbols, data subcarriers are pre-processed prior to the IFFT. According to one method, data symbols are weighted with real-valued numbers. These weights are chosen to reduce the out-of-band emissions caused by the rectangular pulse shape. Due to this weighting, the Bit Error Rate (BER) will increase the more the out-of-band emission is suppressed.
Further, in other prior art solutions belonging to the second category mentioned above, i.e. introducing correlation between transmitted data symbols, cognitive multi-band OFDM systems have been considered where the problem is to achieve low interference in certain parts of the frequency band.
Methods have been proposed, whose purpose is to create frequency notches within the OFDM frequency band, in which other systems could operate. Solutions according to these methods achieve a form of in-band power emission reduction, where the interference in a so called victim-band should be minimized. The unwanted power in the victim-band is due to the finite duration of the OFDM symbols, which translates to an unlimited width of the frequency spectrum, such that the frequency spectrum for OFDM only is zero at the subcarrier frequencies. Hence, there will be undesired in-band power at frequencies located between subcarriers.